$20000 is a
lot of capital; I could probably live for a year on that, and I don't live in a cheap location. A young adult (low medical expenses, no children, no mortgage) living in a less-populated area (north pacific, midwest, south) could probably stretch that to two or even three years, if they find a decent deal on an apartment.
Classes at the local community college run about $50 per quarter, plus books. Building a website is free, and hosting it nearly so. Marketing doesn't have to mean full-page ads in the New York Times -- a few humble links on popular community sites can go a long way.
I think there's a certain group mentality on News.YC which builds up the idea of startups as hugely capital-intensive operations, requiring hundreds of thousands in capital and slick marketing campaigns across the web. This mentality is harmful. Every startup I've ever heard of has either 1) started small and built slowly or 2) flamed out in a hissing ball of comedy. There's no doubt thousands I've not heard of, which quietly curled up in a corner and died after spending all their investments on fancy chairs and television commercials.